I agree, it's all about how you feel. I know for me personally I have a big problem taking breaks. I always work and push myself to failure every time I work out. I raced dirt bikes for a living for a long time and I think because with racing it is a year round thing, I am used to training year round with no breaks so I do the same thing now that I am lifting to get bigger. I try to take a break before I feel I need too, that way I always have the drive to keep working. The problem I had before with racing is the only time I would take a break is when I got injured, and when you come back from an injury you have that killer drive back and then some.